Accurate measurements of chemical reaction rates can only be accomplished with data obtained from reactors operated at two specific extremes of mixing. The first of these extremes is referred to as a plug flow reactor. Plug flow reactors are sometimes referred to as slug flow, piston flow, tubular flow or non-backmixing flow reactors. These reactors are characterized by the fact that flow of fluid through the reactor is orderly with no element of fluid overtaking any other element. Consequently, no diffusion along a flow path and no difference in velocity for any two elements of flowing fluid are permitted. The second of these extremes is the continuous stirred tank reactor. Continuous stirred tank reactors are sometimes referred to as backmix, total backmix or constant flow stirred tank reactors. In these reactors, the contents are well stirred and uniform in composition throughout. Thus the exit streams from these reactors have the same composition as the fluid within them. Reactions performed in a continuous stir tank reactor can be evaluated with relatively simple, algebraic equations. Reactions performed in a plug flow reactor can be evaluated with more complex mathematics, involving integration of space/time relationships. Reactors which do not approximate either of these mixing extremes can only be evaluated with the use of a great number of assumptions which leads to inherent uncertainties.
To avoid these uncertainties, large commercial size reactors are usually operated at high throughputs. These conditions usually approximate plug flow conditions. It is these conditions which permit, for example, the scale-up of pilot-plant data to commercial size reactors. The achievement of plug flow conditions in small laboratory reactors is, however, very difficult and usually impossible.
The difficulty with achieving plug flow conditions in small laboratory reactors has led to the search for laboratory reactors which can be operated as continuous stirred tank reactors. From such continuous stirred tank reactors, kinetic parameters can be accurately obtained. The data obtained from such continuous stirred reactors can be used in plug flow equations, permitting design of commercial-size reactors operated in the conventional configuration.
The simplest form of a continuous stirred reactor is an empty vessel in which two gaseous reactants are introduced to form a single product. All three fluids are continuously well mixed due to the limited mass transfer resistance exhibited.
The mixing of two miscible liquids with similar density is slightly more difficult due to the lower diffusivities of liquids relative to gases. However, if the liquids are of significantly different densities or are not miscible, intense stirring by mechanical means is necessary to prevent the pooling of the heavier liquid phase on the bottom of the reactor. Such segregation tends to inhibit attempts to operate the reactor as a continuous stirred tank reactor.
Heterogeneous systems containing liquids, gases and solids (e.g., solid catalysts) are significantly more complex. A simple slurry of a catalyst in liquid phase is possible if the catalyst density is approximately the same as that of the liquid. However, special efforts must be taken to assure the absence of catalyst stratification in the liquid phase and to provide a means of gas phase draw-down into the liquid phase. The presence of gas bubbles in the liquid phase often decreases the apparent density of the liquid phase relative to the catalyst density and leads to catalyst segregation at the bottom of the reactor. Consequently, mechanical means, such as catalyst baskets, are often used to support catalysts.
A number of laboratory reactor designs have been tested with the goal of achieving continuous stirred tank reactor performance. One of these designs involves different configurations of rotating catalyst baskets immersed in the fluid phase. Another of these designs has a centrally-located fixed bed of catalyst and internally recycles the fluid phases through it. Still another design places the catalyst in an annular fixed bed. The fluid is circulated through the bed by the flat vanes of a centrally located impeller. The fluid then flows vertically along the reactor walls and is directed back to the reactor center by pitched impellers located above and below the flat impeller on the same rotating shaft. A problem that has plagued each of these designs is the formation of pockets of stagnant fluid which is detrimental to obtaining continuous stirred tank reactor performance.
It would be advantageous to provide a mixing apparatus suitable for achieving continuous stirred tank reactor performance. It would be particularly advantageous if such continuous stirred tank reactor performance could be achieved when multi-phase mixing is required.